In recent years contamination of soil and ground water with heavy metals has been identified as a serious environmental hazard. Heavy metals are known to be toxic to most wildlife and man in relatively low concentrations. Elements such as lead, platinum, mercury, cadmium, cobalt, zinc, tin, arsenic, and chromium are used in many industrial applications and often significant levels of these metals are found in industrial waste streams. Heavy metals are also found in organic form such as organoarsenic and organotin, used as pesticides or herbicides, as well as nickel tetracarbonyl and tetraethyllead produced as by-products of the petroleum industry.
A number of methods have been described to remediate soil and ground water containing toxic chemicals. These methods primarily focus on concentration and removal or containment of contaminated media or on the use of microbes to enzymatically transform toxins to inert forms. Revis et al. (U.S. Pat. No. 4,826,602) claim that contacting aqueous waste with a culture of Pseudomonas maltophilica ATCC 53510 will reduce the concentration of ionic species of heavy metals. Colaruotolo et al. (U.S. Pat. No. 4,511,657) teach the use of specially adapted microbial cultures to treat obnoxious waste, especially halogenated organic chemical waste (U.S. Pat. No. 4,493,895).
The use of bioreactors and in-situ stimulation of indigenous microflora are two current approaches to the decontamination of soil and ground water. Bioreactors have been designed to utilize microorganisms for the bioremediation of a variety of toxic contaminants, including trichloroethylene, phenol and toluene. (Folsom et al. 1991 Applied and Environmental Microbiology. 57:1602-1608). In-situ bioremediation involves the growth of indigenous, contaminant-degrading microorganisms which are enhanced by adding nutrients and oxygen. Raymond (U.S. Pat. No. 3,846,290 and U.S. Pat. No. 4,588,506) teaches a process in which oxygen and nutrients are supplied to biota for stimulating the biooxidation of hydrocarbons contaminating ground water without the addition of microorganisms to the contaminated environment.
The methods cited above are useful and clearly show that microorganisms can be used to remove toxic compounds, from both soil and aqueous environments. There are, however, several disadvantages to the methods outlined in the existing art. Examples given in the art describe decontamination of the environment using specific naturally occurring, or genetically engineered cultures of bacteria or yeast or the preliminary harsh chemical treatment of toxic contaminants prior to biological treatment by indigenous microbes. The isolation or engineering, culturing and inoculation of specific microorganisms particularly selected for the degradation of specific organic contaminants is labor intensive and time consuming. Bioreactors can allow for effective microbial growth with greater control over nutrient addition, temperature, pH, and concentration, however, in bioremediation projects, materials must be pumped out or excavated and soils must be handled and sorted which is also labor intensive. Bioremediation efforts that utilize in-situ methods have been effective in degrading certain toxic compounds, however, they have not addressed the specific problem of metal and organometal contamination. Many of the problems associated with these techniques have provided the incentive to look to the use of green plants for simpler and more economically attractive means of remediating soil and ground water of heavy metal species.
It has been know for some time that many plant species will concentrate certain metals in their leaves, stems and roots to a varying degree. (Baker et al., Ecophysiology of Metal Uptake by Tolerant Plants In: "Heavy Metal Tolerance in Plants: Evolutionary Aspects" A. J. Shaw (ed.) CRC Press (1989)) teach that a green plant's response to a metalliferous environment ranges from active exclusion of the metallic species to tolerance to accumulation and even hyperaccumulation where concentrations may approach greater than 1% of plant dry matter. The phenomenon of accumulation and hyperaccumulation of metals by plants has been demonstrated over a wide range of plant families and to date it has not been possible to predict which plants of which families will function as metal accumulators and/or hyperaccumulators. Further complicating the issue is the fact that plants that might be classified as hyperaccumulators of one metal species may be barely tolerant of another. Hence the phenomenon is specific not only for plant type but also for metal species. (Baker et al., Ecophysiology of Metal Uptake by Tolerant Plants In: "Heavy Metal Tolerance in Plants: Evolutionary Aspects" A. J. Shaw (ed.) CRC Press (1989)) For example various species of Alyssum are known to be hyperaccumulators of nickle reaching levels of 13400 ugNi/g but do not appear to be hyperaccumulators of other metals. Thlaspi sp. on the other hand demonstrate hyperaccumulation of a variety of metals including nickle, zinc, and lead. To date the plant that has shown the greatest ability to accumulate lead is Thlaspi rotundifloium attaining levels of 8200 ugPb/g dry weight of the plant. (Baker et al., Ecophysiology of Metal Uptake by Tolerant Plants In: "Heavy Metal Tolerance in Plants: Evolutionary Aspects", A. J. Shaw (ed.) CRC Press (1989)) A summary of many of the known hyperaccumulators is included in Baker et. al., Terrestrial high plants which hyperaccumulate metallic elements--a review of their distribution, ecology and phytochemistry, Biorecovery, 1, 81, (1989) herein incorporated by reference.
The work surrounding the studies of accumulation and hyperaccumulation of metals by plants has been focused in the areas of using these plants as indicators of metal contamination and as study models to prevent toxic metal accumulation in food crops. The concept of utilizing the accumulating phenomena as a tool to extract metals from a contaminated environment was discussed by R. L. Chaney, Plant uptake of Organic waste constituents In: "Land Treatment of Hazardous Wastes", Parr et al. (ed.) Noyes Data Corporation New Jersey (1983). Chaney notes that hyperaccumulators of nickle, and copper are known to accumulate these metals to as much as 1% of dry plant weight and suggests that they might be used to bioconcentrate these metals from land treatment sites. Chaney, however does not teach a method for accomplishing the bioconcentration.
Takashi Utsunomiya (JP 57000190) teaches the use of various plants including those of the genera Polygonaceae, to remove heavy metals and particularly cadmium and mercury from contaminated soil by the steps of cultivating the plants in the contaminated soil, and removing the plants from said soil after the plants have reached a certain stage of growth. Utsunomiya also teaches a poorly defined link between the presence of certain glycoside compounds in these plants and their ability to accumulate the desired metals. Utsunomiya also anticipates the use of these plants in hydroponic systems to remediate aqueous environments of metal contamination. The invention of Utsunomiya does not teach the use of these plants to concentrate organic or inorganic lead species. Furthermore the plants used by Utsunomiya accumulate metals only to levels of less than 100 ppm, putting them outside the class of plants considered to be significant metal accumulators and calling into question the practical utility of these plants to concentrate toxic metals.
M. Rogmans (DE 3921336) also teaches a process for the use of Polygonum sp. to remediate soil of liquid soluble contaminants including cadmium, lead, and zinc. Rogmans also teaches the production of a heavy metal resistant strain of Polygonium sp. via the selection of high metal capacity cell lines and the regeneration of these cells to form a new, high metal resistant plant. Rogmans does not teach the use of Ambrosia sp. or Apocynum sp. for this purpose.
Menser, H. A. et al., Environmental Pollution 18(2), 87-95, (1979) describe the analysis of several plants, including common ragweed (Ambrosia artemisiifolia) and smartweed (Polygonum pennsylvanicum) isolated from a municipal landfill for concentrations of various heavy metals including Mh, Cu, Co, Cr, and Pb. The highest concentration of Pb that was recorded for either plant was 3.68 ppm dry weight of the plant. Mense does not teach accumulation or hyperaccumulation of lead by ragweed or dogbane.
Ideally, requirements for a plant to be used for the purpose of remediating soil, water, and other contaminated media from heavy metals would be that it should be an accumulator of the desired heavy metal (i.e., be able to accumulate levels of at least 1000 mg/kg in the above ground tissues), be a hardy plant that will withstand a broad range of weather and environmental conditions and that it be fast growing to permit harvesting of several crops per growing season. Additionally, some benefit may be accrued by the plant being easily adapted to hydroponic growth conditions. It is the object to the present invention to provide the members of the genera Ambrosia (ragweed) and Apocynum (Dogbane) as such plants and to provide a process for the remediation of contaminated soil, water and other media of lead and organolead compounds.